Aliskiren is a renin inhibitor which is under development for the treatment of several cardiovascular indications including congestive heart failure, hypertension and chronic renal failure. The renin-angiotensin cascade is one of the key regulators of electrolyte and fluid balance and blood pressure. Currently available drugs, including ACE inhibitors (angiotensin converting enzyme) are antagonists of the latter parts of this cascade. Aliskiren, the first of a new class of peptidomimetics, offers potential as a more selective inhibitor through action in the first half of the pathway.

Aliskiren, also known as SPP-100, can be synthesised through the addition of a Grignard reagent derived from a 3-aryl-2-isopropyl-1-chloropropane to the nitrone function of a pseudoephidrine spiroanellated gamma-butyrolactone (Dondoni et al., Tetrahedron Lett., 42 (2001), 4819-4823). A more efficient synthesis, described in WO 01/09079 and WO 02/092828 proceeds via an (S)-5-chloro-2-isopropylpent-4-enoic-acid alkyl ester (1). An ester of formula (1) can be obtained in enantiomerically enriched form by hydrolysis of corresponding racemic ester to form the carboxylic acid, followed by racemate separation by means of diastereomeric salt formation with chiral amine bases and crystallization, followed by re-esterification. A stereoselective synthesis utilising chiral auxiliaries is further described for the preparation of such 2(S)-pentenoic acids and their derivatization to form the corresponding carboxylic acid halides, esters and amides. This stereoselective synthesis of the esters is not yet satisfactory and is regarded as too costly. WO 02/092828 describes how an ester of formula (1) can be obtained preferentially, by bioresolution of the racemic carboxylic ester (2) using a mammalian esterase, namely Pig liver esterase (PLE). Although from a technical perspective this provides an efficient process, for the reasons set out below use of a mammalian esterase in commercial production of the key intermediate (1) is not favoured. Furthermore, to overcome to this problem, it is normally very difficult to identify suitable non-mammalian alternatives to mammalian enzymes that provide equivalent catalysis for such preparative bioresolution processes.
There are a number of reasons that the use of animal derived products in the synthesis of pharmaceutical intermediates is commercially unattractive. Principal among these is the fact that they are becoming subject to increasingly stringent regulatory guidance. The “note for guidance on minimizing the risk of transmitting spongiform encephalopathy agents via human and vetinary medicinal products” (EMEA/410/01/01 Rev2 October 2003) became enforceable as of 1 Jul. 2004 The driving force behind such scrutiny is the increasing incidence of previously unidentified emergent diseases such as the transmissible spongiform encephalopathy responsible for BSE (bovine spongiform encephalopathy). Such emergences, like the virus causing SARS (Severe Acute Respiratory Syndrome) arising from organisms jumping the cross species barrier are also a cause for future concern. In such cases, and other highly infectious animal diseases, for example, Foot and Mouth disease, quarantine and mass culling of animals has resulted, and this could have serious impact upon the supply of materials derived from animals susceptible to infection. In response, many pharmaceutical companies are taking pre-emptive action and require processes for manufacture of active pharmaceutical ingredients (APIs) and their registered starting materials to be developed without animal products. This mitigates the risk of later being held responsible for any prevailing patient infections and moreover protects the supply chain of raw materials used in the manufacture of APIs.
For investigation of enantioselective hydrolysis processes for carboxylic esters, the range of commercially available esterases is limited. Most come from animal origin, for example, pig liver esterase (PLE). Attempts have recently been reported to produce recombinant PLE in a heterologous bacterial host (WO2004/055177). However, since the selectivity of native PLE preparations may derive from a particular combination of isoforms present in the animal derived product, such artificial esterases may not always have the desired selectivity for a given reaction.
Politino et al. (Appl Environ Microbiol. 1997 63(12):4807-11; U.S. Pat. No. 5,869,309) describe the use of Rhodosporidium toruloides as a source of a cephalosporin esterase, having utility in the hydrolysis of the 3′-acetyl group of certain cephalosporins.